1) Field of the Invention
The present invention relates to a composite magnetic member comprising at least one non-magnetized part and at least one ferromagnetized part, both parts being continuously and integrally formed, a process for producing the member and an electromagnetic valve using the member, and particularly to a composite magnetic member capable of fully maintaining ferromagnetic and non-magnetic characteristics against changes in temperature, a process for producing the member and an electromagnetic valve using the member.
2) Description of the Related Art
In generating a magnetic circuit in products such as electromagnetic valves, etc. by discretely providing a ferromagnetic part and a non-magnetic part in one product, it is necessary to make a mild steel part from a ferromagnetic material and an austenitic stainless steel part from a non-magnetic material individually, then assemble the ferromagnetic part and the non-magnetic part together while joining the parts properly by bonding, for example, by soldering, to make a member for the magnetic circuit. However, in making a member for the magnetic circuit in this manner, it is necessary to make a plurality of parts from a ferromagnetic material and a plurality of parts from a non-magnetic material individually and assemble such pluralities of the parts, while joining them by bonding. Thus, many steps and much labor are required for making such a member, complicating the procedure.
It is known that ordinary austenite stainless steel, high manganese steel, etc. are in a non-magnetic state after solid-solution treatment, but can be given a ferromagnetic property by cold working at room temperature to induce and generate a martensite structure. However, the degree of ferromagnetization obtained by this procedure is not high and thus it is actually difficult to apply this procedure to the production of members for the magnetic circuit.
It is also possible as a means for locally non-magnetizing part of a ferromagnetic material such as mild steel, etc. to diffuse an austenitizing element such as Mn, Cr, Ni, etc. into the ferromagnetic material from the surface, but such a means still has a problem in the production of members for the magnetic circuit.
JP-A-63-161146 discloses materials utilizable as a magnetic scale by optimizing the composition of austenite stainless steel or high manganese steel and working procedures for such materials to make members having both ferromagnetic and non-magnetic properties at the same time, where metastable austenite stainless steel is cold drawn into wires, thereby ferromagnetizing the austenite stainless steel based on martensitizing of austenite structure and part of the martensitized wires are further subjected to a local solid-solution treatment to locally non-magnetize the martensitized wires on the basis of local back-austenitization. Members having both ferromagnetic and non-magnetic properties at the same time can be obtained thereby. In this case, the composite magnetic members disclosed in JP-A-63-161146 can have a satisfactorily ferromagnetized part and a satisfactorily non-magnetized part, as integrated together, which can work satisfactorily under ordinary circumstances, but no measures have been taken against temperatures at which the non-magnetized parts are to be used. That is, under severe temperature circumstance such as an extremely low temperature circumstance, a martensite structure is generated on the non-magnetized part, thereby transforming the non-magnetic properties to ferromagnetic properties. This has been a problem.
Currently available electromagnetic valves work as follows: a magnetic circuit is generated by passing an electric current through a coil in the valve, and a plunger is actuated through a sleeve undergoing magnetic working by the generated magnetic circuit. Particularly when an electromagnetic valve is used for oil-hydraulic control, the plunger must slide oil-tight along the inside surface of the sleeve. The conventional sleeve is made from a non-magnetic material, and to make the plunger behavior more sensitive, the magnetic circuit must be permeated through the non-magnetic material, and thus the force of excitation of the coil itself must be increased. Still furthermore, it is possible to ferromagnetize only part of the sleeve through which the magnetic circuit is to permeate. In the sleeve structure made by integrating a plurality of parts by bonding, the bonding must be carried out by soldering, welding, or the like to make the sleeve, and thus considerable post-working is required for obtaining desired dimension, shape and precision. Thus, there is a post-working problem.